Electrophotography forms the technical basis for various well known imaging processes, including photocopying and some forms of laser printing. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoconductor element, imagewise exposing the photoconductor element to activating electromagnetic radiation, also referred to herein as "light", thereby dissipating the charge in the exposed areas, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoconductor element to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material.
The structure of a photoconductor element may be a continuous belt, which is supported and circulated by rollers, or a rotatable drum. All photoconductor elements have a photoconductive layer which conducts electric current when it is exposed to activating electromagnetic radiation. The photoconductive layer is generally affixed to an electroconductive support. The surface of the photoconductor is either negatively or positively charged such that when activating electromagnetic radiation strikes the photoconductive layer, charge is conducted through the photoconductor in that region to neutralize or reduce the surface potential in the illuminated region. An optional barrier layer may be used over the photoconductive layer to protect the photoconductive layer and extend the service life of the photoconductive layer. Other layers, such as adhesive or priming layers or charge injection blocking layers, are also used in some photoconductor elements.
Typically, a positively charged toner is attracted to those areas of the photoconductor element which retain a negative charge after the imagewise exposure, thereby forming a toner image which corresponds to the electrostatic latent image. The toner need not be positively charged. Some toners are attracted to the areas of the photoconductor element where the charge has been dissipated. The toner may be either a powdered material comprising a blend of polymer and colored particulates, typically carbon, or a liquid material of finely divided solids dispersed in an insulating liquid. Liquid toners are often preferable because they are capable of giving higher resolution images.
The toner image may be transferred to the substrate or an intermediate carrier by means of heat, pressure, a combination of heat and pressure, or electrostatic assist. A common problem that arises at this stage of electrophotographic imaging is poor transfer from the photoconductor to the receptor. Poor transfer may be manifested by low transfer efficiency and low image resolution. Low transfer efficiency results in images that are light and/or speckled. Low image resolution results in images that are fuzzy. These transfer problems may be alleviated by the use of a release coating.
The release layer is applied over the photoconductive layer or over the barrier layer if a barrier layer is being used. The release layer preferably adheres well to the photoconductive or barrier layer without the need for adhesives. Moreover, the release layer must not significantly interfere with the charge dissipation characteristics of the photoconductor construction.
Typical release coatings known in the electrophotographic arts include silicone polymers such as those disclosed in U.S. Pat. No. 4,600,673. Conventional siloxane release materials tend to swell signficantly in the hydrocarbon solvents which are used as carrier liquids in electrophotography. Swollen polymers generally have reduced toughness, and siloxanes, which typically do not have good tensile properties, are very easily scratched when swollen.
There is increasing demand for more imaging cycles per photoconductor construction. Thus, a more durable release layer is desired. Specifically, the release layer should be mechanically durable to withstand abrasion of the various rollers and scrapers which contact the photoconductor element. The release layer must also be resistant to toner carrier liquids.